1. Field of the Invention
The present invention relates to an electronic device such as a note-book type personal computer and others.
2. Description of the Related Art
The note-book type personal computer includes: a body having electronic components such as CPU and others and also having a keyboard; and a cover pivotally connected to the body. The cover includes: a display element such as a liquid crystal display element; and a lighting device for lighting the display element. The lighting device includes: a light guide plate; a discharge tube (cold cathode ray tube) arranged on a side of the light guide plate; and a reflector.
A problem is caused in which a luminous intensity of the discharge tube is lowered even when the liquid crystal element has not come to the life. The reason is described as follows. Liquid mercury charged in the discharge tube is collected at positions close to the electrodes arranged at both end portions of the discharge tube. Metal of the electrodes of the discharge tube is spattered by electrons generated in accordance with discharging, and particles of metal, which have been spattered in this discharging process, attach to a tube wall of the discharge tube as if the particles of metal covered the liquid mercury. Therefore, although a sufficiently large quantity of mercury has been charged into the discharge tube, a quantity of mercury contributing to the emission of light is reduced. Therefore, the luminous intensity of the discharge tube is lowered and the life of the discharge tube is shortened.
Originally, portions close to the electrodes at both end portions of the discharge tube are portions where a large quantity of heat is generated and the temperature is high. However, in the structure in which the discharge tube is supported by a reflector through a support member, heat generated by the discharge tube is thermally conducted to the reflector through the support member. Therefore, temperatures of portions close to the electrodes of the discharge tube are lowered. Since liquid mercury is collected to positions in the discharge tube where the temperatures are low, that is, since liquid mercury is collected to positions close to the electrodes at both end portions of the discharge tube, the above problems are caused. Therefore, when a portion except for both end portions of the discharge tube is cooled, for example, a central portion of the discharge tube is cooled, liquid mercury is collected in the central portion of the discharge tube, the temperature of which is low. Therefore, a quantity of mercury contributing to the emission of light is not reduced. Accordingly, luminance of the discharge tube is not reduced and it becomes possible to prevent the life of the discharge tube from being shortened.
For example, there is a proposal in which an air flow is generated in a cover by a natural convection, and a discharge tube is cooled by this air flow. (Concerning this technique, refer to Japanese Unexamined Patent Publication (Kokai) Nos. 61-294528, 4-290107, 5-93908 and 2002-189207.) However, according to this method, a quantity of air in the flow is so small that cooling can not be sufficiently executed. There is another proposal in which a fan is provided in a cover so as to cool a discharge tube. (Concerning this method, refer to Japanese Unexamined Patent Publication (Kokai) No. 10-54985). However, when the fan is arranged in the cover, the cover size is increased. Therefore, it becomes necessary that the entire cover is redesigned.